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## Backreferencing
<html>
<head>
<style type="text/css">
.container {
position: static;
width: 800px;
height: 350px;
overflow: hidden;
}
.embed {
height: 100%;
width: 100%;
min-width: 1000px;
margin-left: -360px;
margin-top: -57px;
overflow: hidden;
}
body {
width: 800px;
margin: auto;
padding: 1em;
font-family: "Open Sans", sans-serif;
line-height: 150%;
letter-spacing: 0.1pt;
}
img {
width: 90%;
text-align: center;
margin: auto;
box-shadow: 0 4px 8px 0 rgba(0, 0, 0, 0.2), 0 6px 20px 0 rgba(0, 0, 0, 0.19);
}
pre, code {
padding: 1em;
}
</style>
<script>
document.addEventListener('readystatechange', event => {
if (event.target.readyState === "complete")
document.activeElement.blur();
});
</script>
Backreferencing is used to match same text again. Backreferences match the same text as previously matched by a capturing group. Let's look at an example:
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuap" class="embed"></iframe>
</div>
**Note:** `\/` is escaped `/`character, check it out in the appendix.
The first captured group is (`\w+`), now we can use this group again by using a backreference (`\1`) at the closing tag, which matches the same text as in captured group `\w+`.
You can backreference any captured group by using `\group_no`.
Let's have two more examples:
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuas" class="embed"></iframe>
</div>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vube" class="embed"></iframe>
</div>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.18.1/styles/default.min.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.18.1/highlight.min.js"></script>
</head>
<body>
### Backreferencing and character class
<h2 id="backreferencing">Backreferencing</h2>
Backreferencing can not be used in character class. Let's see an example:
<p>Backreferencing is used to match same text again. Backreferences match the same text as previously matched by a capturing group. Let's look at an example:</p>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vu7p" class="embed"></iframe>
</div>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuap" class="embed"></iframe>
</div>
### Backreferencing and quantifiers
<p><strong>Note:</strong> <code>\/</code> is escaped <code>/</code>character, check it out in the appendix.</p>
When we are using a backreference for an expression with quantifiers, then we have to be careful. Let's observe it:
<p>The first captured group is (<code>\w+</code>), now we can use this group again by using a backreference (<code>\1</code>) at the closing tag, which matches the same text as in captured group <code>\w+</code>.</p>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vu85" class="embed"></iframe>
</div>
<p>You can backreference any captured group by using <code>\group_no</code>.</p>
Note that `(\d)+` and `(\d+)` both are different. So, what will happen for `(\d)+ -- \1` expression and same text above?
<p>Let's have two more examples:</p>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vu85" class="embed"></iframe>
</div>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vuas" class="embed"></iframe>
</div>
Can you observe something?
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vube" class="embed"></iframe>
</div>
For `(\d)+ -- \1` expression and `123 -- 3` string, first time 1 was stored in \1, then 2 was stored in \1 and at last 3 was stored. So, it will show a match if and only if the last character before ` --` is exactly same as the character after `-- `.
<h3 id="backreferencingandcharacterclass">Backreferencing and character class</h3>
**Problems:**
<p>Backreferencing can not be used in character class. Let's see an example:</p>
1. Match any palindrome string of length 6, having only lowercase letters.
Answer: `([a-z])([a-z])([a-z])\3\2\1`
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vu7p" class="embed"></iframe>
</div>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vubk" class="embed"></iframe>
</div>
<h3 id="backreferencingandquantifiers">Backreferencing and quantifiers</h3>
2. **RegEx**: `(\w+)oo\1le` <br>
**Text:** `google, doodle jump, ggooggle, ssoosle`
<p>When we are using a backreference for an expression with quantifiers, then we have to be careful. Let's observe it:</p>
Answer:
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vubk" class="embed"></iframe>
</div>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vu85" class="embed"></iframe>
</div>
**Note:** For group numbers more than 9, there is a syntax difference.
<p>Note that <code>(\d)+</code> and <code>(\d+)</code> both are different. So, what will happen for <code>(\d)+ -- \1</code> expression and same text above?</p>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vu85" class="embed"></iframe>
</div>
<p>Can you observe something?</p>
<p>For <code>(\d)+ -- \1</code> expression and <code>123 -- 3</code> string, first time 1 was stored in \1, then 2 was stored in \1 and at last 3 was stored. So, it will show a match if and only if the last character before <code>--</code> is exactly same as the character after <code>--</code>.</p>
<p><strong>Problems:</strong></p>
<ol>
<li><p>Match any palindrome string of length 6, having only lowercase letters.
Answer: <code>([a-z])([a-z])([a-z])\3\2\1</code></p>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vubk" class="embed"></iframe>
</div></li>
<li><p><strong>RegEx</strong>: <code>(\w+)oo\1le</code> <br>
<strong>Text:</strong> <code>google, doodle jump, ggooggle, ssoosle</code></p>
<p>Answer: </p>
<div class="container">
<iframe scrolling="no" style="position: absolute; top: -9999em; visibility: hidden;" onload="this.style.position='static'; this.style.visibility='visible';" src="https://regexr.com/4vubk" class="embed"></iframe>
</div></li>
</ol>
<p><strong>Note:</strong> For group numbers more than 9, there is a syntax difference.</p>
<script type="text/javascript">
document.addEventListener('DOMContentLoaded', (event) => {
document.querySelectorAll('pre code').forEach((block) => {
hljs.highlightBlock(block);
});
});
</script>
</body>
</html>